Objective To observe the effect of early mild hypothermia on the opening of tight junction of cerebral endothelial cells following traumatic brain injury in order to illustrate possible mechanism of low permeability of blood brain barrier (BBB) treated by mild hypothermia. Methods A total of 90 Wistar rats were randomly divided into normothermia control group (n=10),normothermia injury group (n=40) and mild hypothermia group (n=40). The opening state and its extent of tight junction were observed using lanthanum trace labeling with electron microscope. At the same time,water content of cerebral tissue at different phases in normothermia injury group and mild hypothermia group was measured by means of dry-wet weight and analyzed statistically. Results The tight junction was under preliminary opening three hours after trauma and reached the maximum opening within 24-48 hours after trauma,lasting for 72 hours in the normothermia injury group. However,slight opening appeared only in the mild hypothermia group. Water content of cerebral tissue in the mild hypothermia group lessened obviously in contrast to that in the normothermia injury group,with significant difference 3,24 and 72 hours after trauma ( P